This invention relates to a glass fiber for optical communication, more particularly, to an improved glass composition for a glass fiber with a large numerical aperture, which can effectively be used for optical communication.
Generally speaking, a glass fiber for optical communication is constituted of a core and a clad layer which is provided around the core and has a smaller refractive index than said core. It transmits an optical information permitted to enter from one end of said fiber to the other end of said fiber by confining the information within said core by utilization of the total reflection at the interface between said core and said clad layer. In the prior art, there have been applied as a glass for such fibers glasses principally composed of quartz (see Japanese Provisional Patent Publication Nos. 60040/1980, 20214/1981, and 222654/1981) and optical glasses in general.
However, in case of the former, there are involved the following drawbacks: p0 (1) Due to the composition predominantly of a single component optically admixed with minute amounts of additives, the refractive index of the central component of quartz is fixed as a physical property value and therefore cannot afford a high refractive index in compliance with the requirement;
(2) In preparation of a fiber by fiber drawing from a rod-shaped mother material glass with a diameter of about 10 mm, if the characteristics of the glass for the core and the glass for the clad layer are greatly changed, that is, the numerical aperture is attempted to be enhanced by giving a greater refractive index difference between both glasses, there will be generated cracks in the mother material due to the difference in thermal expansion coefficient between both glasses resulting in a failure of fiber drawing and giving only limited range of feasible characteristics. PA0 (1') In preparation of an optical glass, there are added considerable amounts of alkali metals or alkaline earth metals to the principal components such as quartz or boric acid, whereby the weathering resistance of the resultant glass is lowered to give characteristics which are not sufficiently reliable under practical conditions; PA0 (2') It is very difficult to remove light absorptive impurities contained in various kinds of starting materials as well as those entrained during the preparation steps, and consequently the resultant fiber is increased in optical loss so as to be not practically useful. PA0 a multi-component system glass for the core having a composition comprising 35 to 45% by weight of SiO.sub.2, 35 to 45% by weight of BaO, 3 to 6% by weight of Al.sub.2 O.sub.3, 9 to 13% by weight of B.sub.2 O.sub.3, 2 to 6% by weight of at least one alkali metal oxide selected from the group consisting of Na.sub.2 O, K.sub.2 O and Li.sub.2 O, the content of Na.sub.2 O being 2.5% by weight or less, and 4 to 11% by weight of at least one alkaline earth metal oxide selected from the group consisting of MgO and CaO; and
On the other hand, optical glasses have the following drawbacks: